Caching

If a user revisits a web page after only a short interval, the page data may not need to be re-obtained from the source web server. Almost all web browsers cache recently-obtained data, usually on the local hard drive. HTTP requests sent by a browser will usually only ask for data that has changed since the last download. If the locally-cached data is still current, it will be reused.
Caching helps reduce the amount of web traffic on the Internet. The decision about expiration is made independently for each downloaded file, whether image, stylesheet, JavaScript, HTML, or whatever other content the site may provide. Thus even on sites with highly dynamic content, many of the basic resources only need to be refreshed occasionally. Web site designers find it worthwhile to collate resources such as CSS data and JavaScript into a few site-wide files so that they can be cached efficiently. This helps reduce page download times and lowers demands on the web server.
There are other components of the Internet that can cache web content. Corporate and academic firewalls often cache web resources requested by one user for the benefit of all. (See also Caching proxy server.) Some search engines, such as Google or Yahoo!, also store cached content from web sites.
Apart from the facilities built into web servers that can determine when files have been updated and so need to be re-sent, designers of dynamically-generated web pages can control the HTTP headers sent back to requesting users, so that transient or sensitive pages are not cached. Internet banking and news sites frequently use this facility.
Data requested with an HTTP 'GET' is likely to be cached if other conditions are met; data obtained in response to a 'POST' is assumed to depend on the data that was POSTed and so is not cached.